1. Field of the Invention
The present invention relates to a wireless selective call receiving apparatus and a wireless selective call receiving method. More particularly, the present invention relates to a direct conversion type of a wireless selective call receiving apparatus and a wireless selective call receiving method to optimally control an operation current of mixers based on a carrier frequency from carrier frequency information stored in a memory.
2. Description of the Related Art
A conventional paging receiver includes a locally oscillating circuit, a mixing circuit and a band pass filter. The mixing circuit mixes a constant frequency signal generated from the locally oscillating circuit with a received signal. The band pass filter is provided in a subsequent stage of the mixing circuit. A frequency band of a signal to be passed through the band pass filter is fixed and narrow. In the paging receiver, only a received signal is demodulated which has a predetermined frequency difference with the frequency signal outputted by the locally oscillating circuit.
In the paging system, a transmission frequency is slightly changed every paging service area to avoid a cross talk among service areas adjacent to each other. For the reason, a paging receiver must have a locally oscillating circuit to output a transmission frequency signal used in a paging area in which the paging receiver is used.
Therefore, many types corresponding to respective transmission frequencies must be manufactured, which cause the manufacturing management to be complex and also causes the drop of the manufacturing cost to be impeded.
In a method disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-215323), a PLL synthesizer 15 is used as a locally oscillating circuit of a receiving section. And a frequency division number N of a programmable frequency divider 20 for determining an oscillation frequency of the PLL synthesizer 15 is set based on information corresponding to a frequency signal to be received stored in an ROM 21.
The locally oscillating circuit is used to send a locally oscillating signal to a mixer. The mixer used in a receiver is operated at a high frequency, and typically requires a large operation current.
Then, a frequency converting apparatus for sending a current to a mixer based on a frequency signal used in the mixer is disclosed in, for example, Japanese Laid Open Utility Model Application (JPU-A-Showa 63-90316).
In the frequency converting apparatus, a high frequency signal and a locally oscillating signal are sent to a transistor of the mixer to be mixed with each other. The frequency converting apparatus has a matching coil to establish a matching among an input end of the mixer, an end for outputting the high frequency signal and an oscillating device for outputting the locally oscillating signal. Moreover, the frequency converting apparatus has a device for switching an operation current used in the transistor between a case of receiving a high frequency band signal and a case of receiving a low frequency band signal. The device has the function of smoothing a frequency characteristic of a conversion gain of the mixer.
However, a wireless selective call receiving apparatus of a direct conversion type needs a mixer to convert a high frequency signal onto a lower frequency signal. The mixer needs a large operation current to operate in the high frequency band. Thus, it is necessary to increase the operation current for the operation frequency to be set to be higher.
Even if the frequency converting apparatus mentioned-above is applied to the wireless selective call receiving apparatus, the frequency converting apparatus does not directly reduce the current consumption of the mixer. This is because the frequency converting apparatus is intended to smooth the frequency characteristic of the conversion gain.
In the wireless selective call receiving apparatus, direct conversion receiving method is effective in miniaturizing the circuit portion. In a normal super heterodyne manner, a band pass filter such as a ceramic filter is needed to process an IF (intermediate frequency) band signal. The size of the band pass filter is one of obstacles to the miniaturization.
On the contrary, a direct conversion receiving circuit processes a locally oscillating frequency signal equal to a carrier wave frequency of an input signal to directly convert a high frequency signal into a base band frequency signal. In this case, a filter to select a desirable signal may be a low pass filter. Thus, it is easy to integrate the direct conversion receiving circuit as an LSI. Therefore the receiving circuit can be miniaturized and the number of the parts can be reduced, in the direct conversion receiving circuit.
Now, the reason is explained why the operation current needed to operate the mixer is different depending on a processing frequency band (a carrier frequency band) and a larger operation current is required as the frequency of the processing signal is higher, in the direct conversion type wireless selective call receiving apparatus.
FIG. 1 is a block diagram showing a configuration of the direct conversion type receiving apparatus as an example of the wireless selective call receiving apparatus. In FIG. 1, a high frequency signal (a received signal) received at an antenna 501 is amplified by an amplifier 502 to be sent to a first mixer 503 and a second mixer 504.
A locally oscillating signal outputted by a local oscillator 505 is branched into two routes. One is directly sent to the first mixer 503 as a first locally oscillating signal. The other is inputted to a 90-degree phase shifter 506. The inputted signal is shifted in phase by 90degrees in the 90-degree phase shifter 506. The shifted signal is sent to the second mixer 504 as a second locally oscillating signal whose phase is shifted by 90 degrees with respect to the first locally oscillating signal.
A constant current source 515 is provided in the first mixer 503. A constant current is sent from the constant current source 515 to the first mixer 503. A constant current source 516 is provided in the second mixer 504. A constant current is sent from the constant current source 516 to the second mixer 504.
The first mixer 503 mixes the amplified high frequency signal with the first locally oscillating signal. The mixed signal is down-converted (converted the frequency thereof) to be outputted to a first low pass filter 507. The second mixer 504 mixes the amplified high frequency signal with the second locally oscillating signal. The mixed signal is down-converted to be outputted to a second low pass filter 508.
The first low pass filter 507 extracts only a base band signal from an output signal of the first mixer 503 to output an in-phase signal (I signal) to a demodulator. The second low pass filter 508 extracts only a base band signal from an output signal of the second mixer 504 to output a quadrature signal (Q signal) whose phase is different from the in-phase signal (I signal) by 90 degrees, to a demodulator 509.
The demodulator 509 demodulates the received signal based on the in-phase signal (I signal) and the quadrature signal (Q signal), which are respectively sent by the first low pass filter 507 and the second low pass filter 508. The demodulated signal is outputted to a control section 510. The control section 510 processes the signal demodulated by the demodulator 509 based on data such as the oscillation frequency of the local oscillator 105 and an ID address of the wireless selective call receiving apparatus stored in a read only memory 511.
The above mentioned direct conversion type wireless selective call receiving apparatus is typically used in a wide carrier frequency between a 150 MHz band and a 450 MHz band. A mixer is provided in an IC for a direct conversion. The operation current of the mixer is optimized based on the maximum usable carrier frequency in the mixer, for example, the 450 MHz band.
Thus, when the mixer is used at the carrier frequency of the 150 MHz band, the current exceeding the necessary value for being used at the 150 MHz band flows through the mixer. This results in the problem that the mixer current is not optimized.